To cope with the mounting flood of information and data which have to be transmitted over ever increasing distances at growing speed, all of which can be handled only with broadband transmission media that allow the highest transmission rates and yet have the least losses, communication techniques employing glass fibers have been developed which rapidly replace existing electronic communications technology down to the private branch exchange level. Devices in glass fiber technology, therefore, enjoy increasing attention.
The information transmission capabilities of optical fibers are well known in the art, and a very large number of optical devices for the connection of optical fibers to transmitter and receiver, as well as with other optical fibers are known in the art.
Partial power coupling between single-mode fibers, for example, is difficult to realize since the actual fabrication of an evanescent-field type coupler is tedious because the fiber core diameter is only a few micrometers, and the fiber is buried deep inside the cladding which is typically only 0.1 mm in diameter.
In a publication by M. J. F. Digonnet et al., IEEE J. Of Quantum Electronics, Vol. QE-18, No. 4 April 1982, pp. 746-754; a tunable single-mode coupler is described where the two halves of the coupler are placed in a holder provided with micrometric screws that permit one to adjust the mutual position of those halves and with it the strength of the coupling between the fibers. A different type of tunable coupler is described in a publication by G. Fawcett et al., Electron. Lett. 28, No. 11, May 1992, pp. 985-986. In G. Fawcett et al., the coupling strength is controlled by an electric field across an electro-optic polymer that is placed in the evanescent field of an optical fiber. Variation of the electric field causes corresponding variation of the refractic index of the polymer, thus leading to intensity modulation in the fiber or in the light collected from the polymer, at a particular wavelength.
While the tunable single-mode coupler described in the publication by M. J. F. Digonnet et al. above is obviously meant for a single adjustment, i.e., it would be impractical to readjust the setting of the micrometer screw during operation, the publication by G. Fawcett et al. does not show how the optical energy could be coupled between two optical fibers.